Coal is not a uniform substance but rather a mixture of combustible, metamorphic plant remains that varies in both physical and chemical composition. Ash producing substance, sulfur, and other materials may be present in amounts as high as sixty percent.
Coal ash is the residue resulting from non-combustible acidic and basic components in the coal. The major acidic components are SiO.sub.2 and Al.sub.2 O.sub.3. Basic components include magnesium oxide, calcium oxide, sodium oxide, potassium oxide, and hematite (Fe.sub.2 O.sub.3). Acidic components include phosphates and sulfates.
Three types of deposits form in coal fired boilers as a result of burning coal: fusion type deposits, alkali matrix deposits, and phosphatic deposits.
Fusion type deposits are the most common. They result from fusion and adhesion of fly ash to the components of the boiler. The chemical composition is a result of starting coal ash. If particle to particle adherence is weak, buildup will be limited to thin layers. A liquid phase, if present, is more likely to adhere than a solid phase, so low melting point components greatly accentuate deposit buildup.
Alkali matrix or bonded deposits form on superheater or generator tube sections and often limit boiler capacity. Alkali metal sulfates, such as Na.sub.2 SO.sub.4 and K.sub.2 SO.sub.4, act to cement fly ash to the metal surfaces.
Phosphatic deposits are uncommon as phosphorous is seldom found in coals, other than as additives. They cause an acidic attack on the metal and heavy fouling.
Sulfur is usually present in coal as inorganic combinations such as pyrite or marcasite or calcium sulfate. The three major forms are organic, pyritic, or sulfate sulfurs.
In almost all cases, it is desirable to clean coal of the majority of the ash and sulfur and other foreign matter prior to transporting and burning the coal. The reasons for doing so include environmental factors such as the contamination of the air and the furnace by pollutants, economic considerations such as the cost of hauling unusable material over extended distances and the elimination of the need for the extremely expensive refitting of coal plants with emission controls, and limitations on the amount of foreign materials which can be tolerated in the process in which the coal is to be used. These savings must be balanced against the cost of cleaning the coal.
Traditionally most coal has been cleaned by crushing and washing with water. Another method is flotation. Flotation processes impart hydrophobic properties to the sulfur and other impurities in crushed coal so that when air is bubbled through the coal suspension, the coal floats to the top. Much work has been done in the area of flotation separation, as shown in U.S. Pat. No. 4,173,530 to Smith et al. Disadvantages and problems associated with flotation separations include limitations on the size of the coal which can be processed, use of specific flotation solutions, poor separations, and fouling of the separation fluids which must then be replaced.
Other cleaning techniques include gravity separations using centrifugal apparatus, such as the methods disclosed in U.S. Pat. Nos. 2,842,319 and 3,908,912; solvent extractions; and combinations of mechanical and chemical separations. Examples of processes in which the coal particles are coated with separation enhancing chemicals are disclosed in U.S. Pat. Nos. 3,852,182 and 3,856,675, which teach coating liquified coal with aromatic compounds such as anthracene, naphthalene or benzene; and U.S. Pat. No. 2,346,151 which discloses slurried coal mixed with an acidic wetting and frothing agent and an antioxidant for separation.
A number of methods have been developed to chemically extract minerals from coal such as is disclosed in U.S. Pat. Nos. 4,045,092 and 4,055,400. The '400 method uses heated alkali solutions to leach out pyritic, organic, sulfur and ash components. The '092 method involves adding calcium hydroxide to fix sulfur in a methanol-coal slurry. Methanol slurries have become relatively widely used. Methanol has certain advantages over both oil and water in transporting micronized coal. Articles describing alkali extraction of methanol treated coal include "Solvolysis Extraction of Assam Coal", Indian Journal of Technology, 20: 235-236 (1982) and "Alkali Treatment of Coal", India Engineering and Chemical Products Resource Development, 22: 488-491 (1983).
Leaching of minerals from coal by an acid has also been used. See "The Effect of Acid Treatment on Dilatometric Properties of Coal", Fuel, 62(1); 5-9 (1983) and "Products of the Treatment of Fusinitic Brown Coals with Mineral Acids and Of Extractions with Ethanol-Benzene", Solid Fuel Chemistry, 16(2): 5-9 (1982).
These processes have failed to combine both commercial feasibility and ash yields of less than 1 w/o. In addition, most cleaning processes alter the combustibility, handling properties and surface chemistry characteristic from that of the starting coal.
It is therefore an object of the present invention to provide a process for deashing coal that is economically and technologically practical.
It is a further object of the present invention to provide a process that yields coal with an ash content of less than one weight percent.
It is a still further object of the present invention to provide a coal with combustibility, manageability and surface chemistry characteristics which are substantially the same as those of the starting material.
It is yet another object of the invention to provide a process for the deashing of coal in which the reagents may be recovered and reused.